Barium Dibenzopentalenide as a Main-Group Metal η8Complex: Facile Synthesis from 1,4-Dilithio-1,3-butadienes and Ba[N(SiMe3)2]2, Structural Characterization, and Reaction Chemistry

Cations and Anions of Dibenzo[a,e]pentalene and Reduction of a Dibenzo[a,e]pentalenophane

Dibenzo[a,e]pentalene (DBP) is a non-alternant conjugated hydrocarbon with antiaromatic character and ambipolar electrochemical behavior. Upon both reduction and oxidation, it becomes aromatic. We herein study the chemical oxidation and reduction of a planar DBP derivative and a bent DBP-phane. The molecular structures of its planar dication, cation radical and anion radical in the solid state demonstrate the gained aromaticity through bond length equalization, which is supported by nucleus independent chemical shift-calculations. EPR spectra on the cation radical confirm the spin delocalization over the DBP framework. A similar delocalization was not possible in the reduced bent DBP-phane, which stabilized itself by proton abstraction from a solvent molecule upon reduction. This is the first report on structures of a DBP cation radical and dication in the solid state and of a reduced bent DBP derivative. Our study provides valuable insight into the charged species of DBP for its application as semiconductor.

Electronic Push-Pull Modulation by Peripheral Substituents in Pentaaryl Boroles

Establishing access to a bulky tetraaryl dilithiobutadiene (Ph*C)₄ Li₂ (Ph*=3,5-tBu₂ (C₆ H₃ )) allowed for the synthesis of five-membered heterocycles with incorporated main-group elements. Along with an amino borole, a set of substituted pentaaryl boroles (Ph*C)₄ BAr has been synthesized. The examination of their absorption spectra and computational studies by means of DFT granted insight into the influence of peripheral substituents on the electronic features of the parent pentaphenyl borole (PhC)₄ BPh. Introduction of the more electron-rich Ph* residue at the carbon atoms increases the HOMO energy, redshifting the visible π/π*-absorption bands compared with the parent pentaphenyl borole. The influence on the frontier orbitals of three different boron-bound aryls with electronically modulating substituents in the remote 3,5-positions Ar=3,5-R₂ -C₆ H₃ (R=Me, H, CF₃ ) was studied. The substituents were found to increase (+I effect, Me) or decrease (-I effect, CF₃ ) the LUMO energy, thus directly affecting the visible absorption spectra. This represents the first study on HOMO-LUMO-gap adjustments by a combined push-pull approach of a substituted pentaphenylborole.

Reactions of Dilithium Dibenzopentalenides with Cr(CO)3 (CH3 CN)3 : Unexpected Formation of a Cubic Tetramer of an Anionic Hydrodibenzopentalenyl Complex

To explore the coordination chemistry of dibenzopentalene dianion, reactions of two dilithium dibenzopentalenides having different silyl substituents with Cr(CO)₃ (CH₃ CN)₃ were investigated. The products were unexpected anionic complexes, [Li(Et₂ O)]⁺ [Cr(η⁵ -9-hydrodibenzopentalenyl)(CO)₃ ]^- . The proton at the 9-position is derived from Cr(CO)₃ (CH₃ CN)₃ , as evidenced by the use of Cr(CO)₃ (CD₃ CN)₃ . The X-ray diffraction analysis revealed that the chromium is coordinated by an anionic five-membered ring of the pentalene skeleton, and the lithium atom is coordinated by oxygen atoms of the carbonyl groups. The complexes form a dimer or a cage-like tetramer via carbonyl-lithium interactions, depending on the bulk of the silyl groups. The cubic tetramer appears to retain its cage structure in nonpolar solvents such as benzene.

The Recently Synthesized Dimagnesiabutadiene and the Analogous Dimetalla-Beryllium, -Calcium, -Strontium, and -Barium Compounds

The 2014 synthesis of the remarkable dimagnesium compound Mg₂ [C₄ (CH₃ )₂ (Si(CH₃ )₃ )₂ ](C₃ H₇ )₂ (C₄ H₈ O)₂ may point the way to a new chapter in alkaline earth organometallic chemistry. Accordingly, we have studied the known Mg compound and the analogous Be, Ca, Sr, and Ba structures. Although most of our theoretical predictions come from density functional methods, the latter have been benchmarked using coupled cluster theory including single, double, and perturbative triplet excitations, CCSD(T) using cc-pVTZ basis sets. Among our most important predictions are the energies for dissociation to the butadiene plus the RM-MR [R=(C₃ H₇ )₂ (C₄ H₈ O)₂ ; M=Be, Mg, Ca, Si, and Ba] entities. The most reliable predictions for the dissociation energies are 99-104 (Be), 85-93 (Mg), 90-99 (Ca), 83-92 (Sr), and 83-94 (Ba) kcal mol^-1 . Thus, there is reason to anticipate that the four unknown compounds should be achievable synthetically. The predicted metal-metal distances (not single bonds) are 2.89 Å (Mg⋅⋅⋅Mg), 3.46 Å (Ca⋅⋅⋅Ca), 3.75 Å (Sr⋅⋅⋅Sr), and 4.04 Å (Ba⋅⋅⋅Ba). The separated RM-MR compounds have longer M-M distances but genuine metal-metal single bonds. This perhaps counter intuitive result is due to the presence of the bridging carbons in the alkaline earth butadiene compounds. All five compounds incorporate metal-carbon ionic interactions.

The B(C6F5)3 boron Lewis acid route to arene-annulated pentalenes

4,5-Dimethyl-1,2-bis(1-naphthylethynyl)benzene (12) undergoes a rapid multiple ring-closure reaction upon treatment with the strong boron Lewis acid B(C6F5)3 to yield the multiply annulated, planar conjugated π-system 13 (50 % yield). In the course of this reaction, a C6F5 group was transferred from boron to carbon. Treatment of 12 with CH3B(C6F5)2 proceeded similarly, giving a mixture of 13 (C6F5-transfer) and the product 15, which was formed by CH3-group transfer. 1,2-Bis(phenylethynyl)benzene (8 a) reacts similarly with CH3B(C6F5)2 to yield a mixture of the respective C6F5- and CH3-substituted dibenzopentalenes 10 a and 16. The reaction is thought to proceed through zwitterionic intermediates that exhibit vinyl cation reactivities. Some B(C6F5)3-substituted species (26, 27) consequently formed by in situ deprotonation upon treatment of the respective 1,2-bis(alkynyl)benzene starting materials (24, 8) with the frustrated Lewis pair B(C6F5)3/P(o-tolyl)3. The overall formation of the C6F5-substituted products formally require HB(C6F5)2 cleavage in an intermediate dehydroboration step. This was confirmed in the reaction of a thienylethynyl-containing starting material 21 with B(C6F5)3, which gave the respective annulated pentalene product 23 that had the HB(C6F5)2 moiety 1,4-added to its thiophene ring. Compounds 12-14, 23, and 26 were characterized by X-ray diffraction.

Pentalenes--from highly reactive antiaromatics to substrates for material science

Antimatter: Once studied primarily for their antiaromatic properties, pentalenes are rapidly becoming important π-systems for novel electronic materials. Recent developments in this area are summarized.